Our purpose is to study the reactions of the reticulo-endothelial system of the rat and other small laboratory animals to prolonged deprivation of magnesium. In the rat the initial reaction is a prompt leukocytosis. This often merges into a malignant lymphoma which arises between the 8th and 20th week of the deficient state in young animals. At a later date (7th to 12th month) a myelogenous leukemia occasionally develops. The myelogenous leukemia rarely occurs in control animals but the lymphoma has not appeared in controls. We want to determine whether the neoplastic reactions are due to a maturation defect induced in stem cells by magnesium deficiency or to an enhancement of cellular susceptibility to tumorigenic viruses or other oncogenic agents, related perhaps to immunologic impairment.